In a communications system including a common sender that has to send data to a plurality of users sharing a common transmission channel, access to the transmission channel may be granted to one user at a time (as in time division multiplex (TDM) systems) or to more than one user at a time (as in code-division multiple access (CDMA) systems). In any event, access to the channel is granted on the basis of certain criteria or scheduling policies. It is important to consider that criteria that enable good performance to be achieved in a cable system may prove unsuited to a wireless system, and vice versa. Among wireless systems, it is also necessary to make a distinction between terrestrial systems and satellite systems, as in these two cases the transmission channel has very different properties.
The paper by Yaxin Cao and V. O. K. Li, “Scheduling algorithms in broadband wireless network”, IEEE Proceedings, Vol. 89, No. 1, January 2001, reviews the criteria for managing user access to the common channel in packet-switched terrestrial wireless networks. The objective of such criteria is generally either to optimize the data rate or to achieve equitable access to resources by the various users. The data rate may be maximized simply by granting access at all times to the user who is experiencing the best channel conditions, that is to say the highest signal to noise plus interference ratio (SNIR). This is no particular problem in terrestrial networks because the channel conditions vary very quickly. Equity may be achieved either in terms of access time to the channel or in terms of the quantity of data transmitted. With quantity of data, it is possible for a user who is experiencing poor access conditions (and therefore a low data rate) to monopolize the channel for a long time, to the detriment of other users. A new approach known as “proportional equity” has recently been proposed for combining data rate maximization with some degree of equity between users. On this topic see the following papers:
“CDMA/HDR: A Bandwidth-Efficient High-Speed Wireless Data Service for Nomadic Users”, P. Bender, P. Black, M. Grob, R. Padovani, N. Sindhusayana, A. J. Viterbi; IEEE Communications Magazine, Vol. 38, No. 7, pp. 70-77, July 2000,
“Data throughput of CDMA-HDR a High Efficiency-High Data Rate Personal Communications Wireless System”, A. Jalali, R. Padovani, R. Pankaj, VTC 2000, and
“Opportunistic Transmission Scheduling with Resource Sharing Constraints in Wireless Networks”, Xin Liu, Edwin K. P. Chong, Ness B. Shroff, IEEE Journal on Selected Areas in Communications, Vol. 19, No. 10, October 2001.
Those techniques prove to be unsuited to satellite communications systems because:                in the case of satellite communications, variations in channel capacity are manifested essentially through intense but sporadic attenuation events that vary slowly and affect a large number of users at the same time, whereas, in contrast, in the case of terrestrial systems, the channel capacity varies quickly and with no correlation between users;        a satellite beam covers a much larger area than a terrestrial system cell;        satellite transmission uses a much wider band than terrestrial transmission (hundreds of Mbps (megabits per second) as against less than 10 Mbps), and a gateway of a satellite communications system must manage a much greater number of packets than its terrestrial counterpart.        
The capacity R of a transmission channel, defined as the maximum information data rate that can be transmitted with an arbitrary low error probability, is given by the following equation, in which B is the bandwidth and SNIR is the signal to noise plus interference ratio:R=B log2(1+SNIR)
Third and fourth generation wireless communications systems routinely use the adaptive coding and modulation (ACM) technique, also known as the adaptive physical layer technique, which consists in modifying the coding and modulation schemes—and therefore the “level of protection” of the data—as a function of the characteristics of the transmission channel. For example, if the SNIR falls, higher channel coding redundancy will be introduced and a lower modulation order (number of bits per symbol transmitted) will be used for transmission, while the symbol data rate remains constant. Accordingly, an acceptable error rate may be maintained despite degraded channel characteristics, at the cost of a reduced information data rate.
Unlike terrestrial systems, satellite communications systems have traditionally been used for broadcasting data. Moreover, conventional systems of this kind are primarily “connection-oriented” (resources are allocated for as long as a query is not blocked), rather than “packet-oriented” like cellular terrestrial systems.
The DVB-S2 standard is a satellite digital communications standard for the Ku (12-14 GHz) and Ka (20-40 GHz) bands, enabling quality of service (QoS) conditions to be satisfied and suited to interactive applications, in particular through the use of the Internet Protocol (IP). It is packet-oriented and encompasses the use of the ACM technology.
The DVB-S2 standard is described in detail in the ETSI document EN 302 307 available from the European Telecommunications Standards Institute (ETSI), 650 Route des Lucioles, F-06921 Sophia Antipolis Cedex, France.
Although the standard as such does not cover scheduling policies, appendix H of the above ETSI document EN 302 307 suggests the possibility of sorting data packets by user, by required service level and/or by protection level (coding and modulation scheme). The first two options are considered in detail in the paper by R. Rinaldo, M. A. Vázquez-Castro, A. Morello, “DVB-S2 ACM modes for IP and MPEG unicast applications”, International Journal of Satellite Communications, No. 22, May 2004.
As is demonstrated below, prior art scheduling policies are unsatisfactory. To be more precise, a first policy based on subdivision of data packets by user and by quality of service (QoS) level is excessively complex and a second policy splitting data packets only by user or by quality of service level is unable to guarantee a minimum data rate for each protection layer and/or to achieve isolation between users. These concepts are essential and consequently need to be defined:                The expression “guaranteed minimum data rate” means that each coding and modulation scheme actually in use at a given time—or only certain of said schemes—is assigned one or more predetermined fractions of the total transmission time. This prevents users experiencing poor channel conditions (and thus a high protection level) from monopolizing the system to the detriment of those experiencing better channel conditions (lower protection level), or vice versa.        The term “equity” means that transmission time is assigned 50% to users experiencing good channel conditions and 50% to users experiencing degraded channel conditions. This concept is therefore a special case of the guaranteed minimum data rate concept.        The term “isolation” means that degraded channel conditions for one user do not reduce much, if at all, the data rate for another user who is not experiencing degraded channel conditions.        